The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube of a shadow mask type for reproducing a color image with a single electron beam.
In a conventional color cathode ray tube, three electron guns are arranged in an in-line- or delta-type electron gun assembly. Electron beams emitted from the respective electron guns are accelerated and converged on a phosphor screen. These electron beams are incident at slightly different predetermined angles on slit apertures of a shadow mask arranged facing the phosphor screen. The electron beams pass through the slit apertures and land on corresponding phosphor stripes, which then emit corresponding color rays. The three electron beams are deflected throughout the entire surface of the screen by a magnetic field generated by a deflection yoke. Therefore, the phosphor screen is scanned with the electron beams. A video signal is applied to the control electrodes of the three electron guns in synchronism with the deflection and scanning of the electron beams, thereby reproducing a corresponding image on the screen.
In a conventional color cathode ray tube of this type, three electron beams must be accurately converged on the front surface of the screen. At the same time, reproduced image distortions such as raster distortion and beam spot distortion caused by electron beam deflection must be eliminated. It is difficult to design a deflection yoke which satisfies the above requirements. In designing a deflection yoke for obtaining good convergence and eliminating distortion, precision depends on the distances between the three electron beams incident on the deflection plane defined by the deflection yoke. The smaller the distances, the easier the design of the deflection yoke. In particular, beam spot distortion can be sufficiently eliminated.
In order to improve resolution, the beam spot size on the screen must be minimized. For this reason, electron lens apertures of the respective electron guns must be maximized.
However, it is impossible to provide both a decrease in distances between the electron beams and an increase in apertures of electron lenses, i.e., the electrode openings of the electron guns.
In a conventional index color cathode ray tube for reproducing a color image with a single electron beam, the electron beam must be converged to form an extremely small beam spot on the screen. Color switching frequencies are high, and the circuit arrangement is complicated. Therefore, such cathode ray tubes are not now widely used.
One proposal was made prior to the widespread application of the current type of color cathode ray tube. According to this scheme, a single electron beam is slightly deflected before the beam reaches the center of deflection. A video signal is switched on the basis of this small deflection, so that three electron beams seem to appear.
According to this system, only one electron beam is required, so the aperture of the electron lens of the electron gun can be increased. However, it is difficult to accurately align the three electron beams on the screen.
In order to solve this problem, U.S. Pat. No. 2,764,628 describes a method for changing the relative relationship between the video signal and the deflection signal to register tricolor images. However, in a system which reproduces each color image with one horizontal scanning, the prior art method cannot completely register tricolor reproduced images.